This invention relates to a print head mounted in an impact type dot matrix printer, and more particularly to a plurality of actuators each having a torsion spring interposed between an armature having a printing wire at its end and a holder retained in the print head.
In each of the actuators of such type of print head, the armature having a printing wire at its end is supported through the torsion spring on the holder retained in the print head, and the armature at the time of printing operation is driven for swinging movement so as to print letters on a paper material with the end of the printing wire. The base end of the armature is located between the bifurcated ends of the holder and the torsion spring inserted in and bonded to the inner walls of through holes provided in both the bifurcated end portion of the holder and the base end of the armature. The torsion spring is formed with larger diameter cylindrical portions in the sections to be bonded to the inner walls of the through holes of both the bifurcated ends of the holder and the base end of the armature, and is formed with smaller diameter cylindrical portions in the sections between these larger diameter portions, with each of the boundary portions between the larger and smaller diameter portions having a little radius of fillet. However when each of the armatures has been swung for printing operation, torsional stress is concentrated in the boundary portions between the larger and smaller diameter portions, which often resulted in breakage of the torsion spring.
On the other hand, as shown in FIG. 7 the end 42a of the armature 42 is made larger in thickness than the axial length of the larger diameter portion 43c of the torsion spring 43. This is because the end 42a of the armature 42 must have a strength which resists dynamic bending applied thereto when the armature 42 has been swung, and also because if the larger diameter portion 43c is made equal in axial length to the thickness of the end 42a of the armature 42, the entire length of the torsion spring 43 becomes larger, which will inevitably result in larger diameter of the print head. Such greater thickness of the end of the armature caused the solder material 44 leaked out at the time of brazing to fill the spaces between the intermediate portions 43c, 43d and the through hole 42b, which in turn caused change in a spring constant of the torsion spring 43. As a result, it was impossible to swing the armature 42 with a substantially uniform torque of the torsion spring.